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Tyrannosaurus
Tyrannosaurus (meaning 'tyrant lizard') is an extinct genus of theropod dinosaur. The famous species Tyrannosaurus rex ('rex' meaning 'king' in Latin), commonly abbreviated to T. rex, is a fixture in popular culture around the world. It lived throughout what is now western North America, with a much wider range than other tyrannosaurids. Fossils of T. rex are found in a variety of rock formations dating to the last four million years of the Cretaceous Period, approximately 70 to 66 million years ago; it was among the last non-avian dinosaurs to exist prior to the Cretaceous–Tertiary extinction event. Like other tyrannosaurids, Tyrannosaurus was a bipedal carnivore with a massive skull balanced by a long, heavy tail. Relative to the large and powerful hindlimbs, Tyrannosaurus forelimbs were small, though unusually powerful for their size, and bore two clawed digits. Although other theropods rivaled or exceeded T. rex in size, it was the largest known tyrannosaurid and one of the largest known land predators, measuring up to 41 feet in length, up to 21 feet tall, and up to 7 metric tons (8 short tons) in weight.Some t.rex's reached up to 10 tons By far the largest carnivore in its environment, T. rex may have been an apex predator, preying upon hadrosaurs and ceratopsians, although some experts have suggested it was primarily a scavenger. More than 30 specimens of T. rex have been identified, some of which are nearly complete skeletons. Soft tissue and proteins have been reported in at least one of these specimens. The abundance of fossil material has allowed significant research into many aspects of its biology, including life history and biomechanics. The feeding habits, physiology and potential speed of T. rex are a few subjects of debate. Its taxonomy is also controversial, with some scientists considering Tarbosaurus bataar from Asia to represent a second species of Tyrannosaurus and others maintaining Tarbosaurus as a separate genus. Several other genera of North American tyrannosaurids have also been synonymized with Tyrannosaurus. Description Tyrannosaurus rex was one of the largest land carnivores of all time; the largest complete specimen, FMNH PR2081 ("Sue") measured 12.3 metres (40 ft) long,3 and was 3.9 metres (13 ft) tall at the hips.4 Mass estimates have varied widely over the years, from more than 7.2 metric tons (7.9 short tons),8 to less than 4.5 metric tons (5.0 short tons),910 with most modern estimates ranging between 5.4 metric tons (6.0 short tons) and 6.8 metric tons (7.5 short tons).5111213 Packard et al. (2009) tested dinosaur mass estimation procedures on elephants and concluded that dinosaur estimations are flawed and produce over-estimations; thus, the weight of Tyrannosaurus could have been much less than usually estimated.14 Other estimations have concluded that the largest known Tyrannosaurus specimens had masses approaching15 or exceeding 9 tonnes.316 The neck of Tyrannosaurus rex formed a natural S-shaped curve like that of other theropods, but was short and muscular to support the massive head. The forelimbs had only two clawed fingers,17 along with an additional small metacarpal representing the remnant of a third digit.18 In contrast the hind limbs were among the longest in proportion to body size of any theropod. The tail was heavy and long, sometimes containing over forty vertebrae, in order to balance the massive head and torso. To compensate for the immense bulk of the animal, many bones throughout the skeleton were hollow, reducing its weight without significant loss of strength. The largest known Tyrannosaurus rex skulls measure up to 5 feet (1.5 m) in length.19 Large fenestrae (openings) in the skull reduced weight and provided areas for muscle attachment, as in all carnivorous theropods. But in other respects Tyrannosaurus's skull was significantly different from those of large non-tyrannosauroid theropods. It was extremely wide at the rear but had a narrow snout, allowing unusually good binocular vision.2021 The skull bones were massive and the nasals and some other bones were fused, preventing movement between them; but many were pneumatized (contained a "honeycomb" of tiny air spaces) which may have made the bones more flexible as well as lighter. These and other skull-strengthening features are part of the tyrannosaurid trend towards an increasingly powerful bite, which easily surpassed that of all non-tyrannosaurids.222324 The tip of the upper jaw was U-shaped (most non-tyrannosauroid carnivores had V-shaped upper jaws), which increased the amount of tissue and bone a tyrannosaur could rip out with one bite, although it also increased the stresses on the front teeth. A 2012 study by scientists Karl Bates and Peter Falkingham suggested that the bite force of Tyrannosaurus could have been the strongest of any terrestrial animal that has ever lived. The calculations suggested that adult T. rex could have generated from 35000 to 57000 newtons of force in the back teeth, or the equivalent of three times the force estimated for a great white shark, 15 times the force of an African lion, 3 1/2 times the force of an Australian saltwater crocodile and around 7 times the estimated force for Allosaurus.2728 However, even higher estimates were made by professor Mason B. Meers of the University of Tampa in 2003. In his study, Meers estimated a possible bite force of around 183000 to 235000 newtons or 18.3 to 23.5 metric tons; a bite force equivalent to that of the largest Megalodon shark specimens.2429 The teeth of Tyrannosaurus rex displayed marked heterodonty (differences in shape).1730 The premaxillary teeth at the front of the upper jaw were closely packed, D-shaped in cross-section, had reinforcing ridges on the rear surface, were incisiform (their tips were chisel-like blades) and curved backwards. The D-shaped cross-section, reinforcing ridges and backwards curve reduced the risk that the teeth would snap when Tyrannosaurus bit and pulled. The remaining teeth were robust, like "lethal bananas" rather than daggers; more widely spaced and also had reinforcing ridges.31 Those in the upper jaw were larger than those in all but the rear of the lower jaw. The largest found so far is estimated to have been 30 centimetres (12 in) long including the root when the animal was alive, making it the largest tooth of any carnivorous dinosaur yet found. Classification Tyrannosaurus is the type genus of the superfamily Tyrannosauroidea, the family Tyrannosauridae, and the subfamily Tyrannosaurinae; in other words it is the standard by which paleontologists decide whether to include other species in the same group. Other members of the tyrannosaurine subfamily include the North American Daspletosaurus and the Asian Tarbosaurus,3233 both of which have occasionally been synonymized with Tyrannosaurus.26 Tyrannosaurids were once commonly thought to be descendants of earlier large theropods such as megalosaurs and carnosaurs, although more recently they were reclassified with the generally smaller coelurosaurs. In 1955, Soviet paleontologist Evgeny Maleev named a new species, Tyrannosaurus bataar, from Mongolia.34 By 1965, this species had been renamed Tarbosaurus bataar.35 Despite the renaming, many phylogenetic analyses have found Tarbosaurus bataar to be the sister taxon of Tyrannosaurus rex,33 and it has often been considered an Asian species of Tyrannosaurus.253637 A recent redescription of the skull of Tarbosaurus bataar has shown that it was much narrower than that of Tyrannosaurus rex and that during a bite, the distribution of stress in the skull would have been very different, closer to that of Alioramus, another Asian tyrannosaur.38 A related cladistic analysis found that Alioramus, not Tyrannosaurus, was the sister taxon of Tarbosaurus, which, if true, would suggest that Tarbosaurus and Tyrannosaurus should remain separate. Other tyrannosaurid fossils found in the same formations as Tyrannosaurus rex were originally classified as separate taxa, including Aublysodon and Albertosaurus megagracilis,26 the latter being named Dinotyrannus megagracilis in 1995.39 However, these fossils are now universally considered to belong to juvenile Tyrannosaurus rex.40 A small but nearly complete skull from Montana, 60 centimetres (2.0 ft) long, may be an exception. This skull was originally classified as a species of Gorgosaurus (G. lancensis) by Charles W. Gilmore in 1946,41 but was later referred to a new genus, Nanotyrannus.42 Opinions remain divided on the validity of N. lancensis. Many paleontologists consider the skull to belong to a juvenile Tyrannosaurus rex.43 There are minor differences between the two species, including the higher number of teeth in N. lancensis, which lead some scientists to recommend keeping the two genera separate until further research or discoveries clarify the situation.3344 Below is the cladogram of Tyrannosauridae based on the phylogenetic analysis conducted by Loewen et al. in 2013. Paleobiology Category:Cretaceous dinosaurs Category:Dinosaurs of North America Category:Tyrannosaurs Category:Geology of Montana Category:Dinosaur genera